1. Field of the Invention
The invention relates to the field of systems that protect living beings from physical harm. More specifically, the invention relates to a circuit for triggering a protection system, in particular, for firing the firing pellet of an airbag system.
Motor vehicle airbag systems usually have firing pellets triggered by electrical current flow. In many instances, a firing capacitor serves as an energy source. Because of the limited energy content of the capacitor, it is necessary to limit the firing current for the triggering situation. European Patent Application EP-A-0 284 728 discloses a system provided with a plurality of airbags using a capacitance situated in the relevant series path to limit the quantity of charge flowing through the series path. If the relevant capacitance is charged, the current flow is interrupted so that the firing pellet of the series path can be fired.
International Patent Application WO 90/02674 discloses an airbag system that limits a current flowing through circuit-breakers in series with a triggering device. Current limiting is achieved by detecting a voltage drop across resistors connected downstream and comparing the voltage drop with reference voltages using a comparator. The comparator provides the circuit-breakers with a signal that is used to limit the level of current flow. This enables the use of one firing capacitor in a known airbag system.
A further measure for current limiting in a circuit for firing an airbag firing pellet is to provide current-limiting switches at the input and at the output of the firing pellet. A high loss of energy is associated with the voltage drop across the switches. Because of this loss of energy, the firing capacitor must be dimensioned to be relatively large, which causes a deterioration of its intended function.
2. Summary of the Invention
It is accordingly an object of the present invention to provide firing current limiting in a protection system circuit, in particular for firing a firing pellet of an airbag system, which is energy-saving and cost-effective.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for firing a firing pellet, comprising:
a switched-mode regulator; and
a low-pass filter connected to the switched-mode regulator;
the switched mode regulator cooperating with the low pass filter to limit an average and a peak value of a current flowing through a firing pellet.
In accordance with an added feature of the invention, the low-pass filter includes an inductor and a capacitor connected to the inductor.
In accordance with an additional feature of the invention, the switched-mode regulator includes a switch that is switched on when the current flowing through the firing pellet falls below a reference value.
In accordance with another feature of the invention, the low-pass filter has two input terminals;
the switched-mode regulator includes a first switch that is switched on when the current flowing through the firing pellet falls below a reference value, the first switch being connected to one of the input terminals of the low-pass filter; and
the switched-mode regulator includes a second switch that is switched on when the current flowing through the firing pellet falls below the reference value, the second switch being connected to the other one of the input terminals of the low-pass filter.
In accordance with a further feature of the invention, the low-pass filter has two input terminals; and the switched-mode regulator includes:
a current sensor having an output providing an indication of the current flowing through the firing pellet; PA1 a comparator having a first input connected to the output of the current sensor, a second input connected to a reference value, and an output; PA1 a first switch connected to one of the input terminals of the low-pass filter, the first switch having a control input; and PA1 a second switch connected to the other one of the input terminals of the low-pass filter, the second switch having a control input; PA1 the output of the comparator connected to the control inputs causing the first and second switches to switch on when the current flowing through the firing pellet falls below the reference value. PA1 a current sensor having an output providing an indication of the current flowing through the firing pellet; PA1 a comparator having a first input connected to the output of the current sensor, a second input connected to a reference value, and an output; PA1 a first switch connected to one of the input terminals of the low-pass filter, the first switch having a control input; PA1 a second switch connected to the other one of the input terminals of the low-pass filter, the second switch having a control input; and PA1 an AND gate having two inputs and an output, one of the inputs connected to a trigger signal; PA1 the output of the comparator being connected to the other of the inputs of the AND gate, the output of the AND gate connected to the control inputs causing the first and second switches to switch on when the trigger signal is active and when the current flowing through the firing pellet falls below the reference value.
In accordance with again an added feature of the invention, the switched-mode regulator includes a comparator with hysteresis.
In accordance with a concomitant feature of the invention, the low-pass filter has two input terminals; and the switched-mode regulator includes:
The invention has the advantage that the average value of the current, including rise and fall times of the current flowing through the firing pellet, is limited by simple circuit elements, namely by a switched-mode regulator in combination with a low-pass filter. This results in a very small voltage drop, and thus a reduced power loss, across the switches provided in the circuit. This provides increased service life of the switches and enables the firing capacitor to have smaller dimensions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit for current limiting in a protection system, in particular an airbag control system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.